Ru(II)-Protic-NHC Complex Catalyzed Selective N-alkylation of Anilines with Primary Alcohols: Mechanistic Insights and Role of Salt Additives

Transition metal complexes with protic-NHC ligands have gained considerable interest, however, their catalytic applications remain underexplored. Herein, the application of Ru complexes with protic-NHC ligands for N-alkylation of amines is reported. A series of phosphine-free Ru(II) complexes (2a-c) with mixed protic- and classical-N-heterocyclic carbenes in benzimidazolylidene-CNC ligands are reported with unsymmetrical, mixed, protic- and classical-NHC ligand CHNCMe. These chiral-at-metal complexes have been characterized as racemic mixtures using multinuclear NMR spectroscopy, high-resolution mass spectrometry, and the solid-state structures of 2a and 2b have been determined using single-crystal X-ray diffraction technique. The protic-NHC complexes 2a-c can be deprotonated to obtain the anionic-NHC complexes (2a-c(y)). Complexes 2a-c show excellent catalytic activity, compared to their imidazolylidene-CNC analogues 3a,b, for N-alkylation of anilines with primary alcohols as alkylating agents, yielding N-alkylated products selectively. Role of simple salt additives, like sodium tetrafluoroborate, for the removal of halide ligand from the coordination sphere and generation of vacant site has been investigated and allowed the catalytic reaction to operate at a lower base loading. Several control experiments and analysis of the catalytic reactions through H-1 NMR and ESI-MS provide important information about a plausible mechanism and confirm the formation of a [Ru & horbar;H] species during the catalysis.